1. Field of the Invention
The present invention relates to an f.theta. lens for converging scanned CO.sub.2 laser beams, irradiating the beams on a printed circuit board and perforating a great number of holes at an ultrahigh speed on the print circuit board by the laser power. The idea of an f.theta. lens itself is not novel. The f.theta. lens means a lens which gives a linear relation h=f.theta. between an incident angle .theta. of the beam and the height h of an image. The proportion constant f is the focal length. But an f.theta. lens for CO.sub.2 laser is a novel concept. Nobody has been aware of the necessity of f.theta. lens for CO.sub.2 laser before this invention. Someone uses the word "laser drilling" for denoting the laser boring technology. In this description, however, we will use "perforation" or "boring" for signifying the laser boring technology for avoiding confusion from the mechanical drilling.
This application claims the priority of Japanese Patent Application No.11-226430(226430/1999) filed on Aug. 10, 1999, which is incorporated herein by reference.
Conventional f.theta. lenses have contributed to laser printers and laser COM systems (computer output microfilm systems). In the laser printers and laser COMs, a laser beam is deflected by a rotating polygon mirror and converged at a point h=f.theta. on a rotating drum by an f.theta. lens. Printer f.theta. lenses have experienced some improvements till now. The f.theta. lens proposed by the present invention is greatly different from the conventional printer f.theta. lens. The light source is a semiconductor laser of e.g., AlGaAs having low power. The light for printers is near-infrared or visible light having a short wavelength. The shortness of the light wavelength allows glass lenses or quartz lenses. The printer f.theta. lens needs no telecentricity because the printer light aims at removing locally electric charges on the drum precoated by a photoconductive material. The printer f.theta. lens features a laser of low power, non-telecentricity and a short wavelength.
The f.theta. lens of the present invention contributes to laser materials processing apparatus having a CO.sub.2 laser or a YAG laser. The CO.sub.2 laser or the YAG laser has far greater power than the printer laser. The wavelength of the CO.sub.2 laser is longer than that of the printer laser. The telecentricity is indispensable for perforating vertical holes by burning the board material through in the vertical direction by the laser beams. The differences between the printer f.theta. lens and the laser processing f.theta. lens will be clarified more in detail later. Nobody has tried to improve an f.theta. lens in the technical field of the laser processing, as long as the Inventor is aware of The laser processing needs no f.theta. lens, since the laser processing has never confronted such a difficult processing as requires an f.theta. lens. The present laser processing technology exploits lenses or mirrors for converging laser rays to a narrow beam. The lenses have been an ordinary f tan .theta. lens which makes a spot distanced from the center by h=f tan .theta., where .theta. is an incident beam angle and f is a focal length. The relation h=f tan .theta. represents quite a normal function of an ordinary lens. The relation h=f.theta. is an extraordinary and artificial property for lenses. The following is the reasons why the Inventor thinks of the necessity of the f.theta. lens in the field of the laser processing technology.
Enhancement of functions and quality of electronic devices requires higher packing density and multilayer structures of printed circuit boards. High-speed, fine perforation technique accompanies the fabrication of printed circuit boards. The holes for mounting devices have been bored on printed circuit boards by mechanical apparatuses for a long time till now. The mechanical apparatus perforates holes by lowering a rotating sharp microdrill on a board, boring a hole, raising the microdrill away, displacing the microdrill in a unit length, lowering the drill at another neighboring spot on the board and repeating the same steps. The mechanical boring has a long achievement. The mechanical perforation, however, has weak points. One weak point is the slow perforation speed, since the microdrill must move in vertical directions and in horizontal directions in a cycle of boring. Another drawback is the point-to-point drilling, since only a single hole is bored at a time by the single microdrill. The other weak point is the limitation of hole size. The requirement of the mechanical strength forbids microdrill from having a diameter less than a definite size. Then, the mechanical boring fails in perforating holes of a diameter smaller than 100 .mu.m.
Attention is paid to laser boring technique for perforating microholes of a diameter less than 100 .mu.m in stead of the mechanical apparatuses. The laser boring processing makes holes by burning the material locally by intense laser power.
The laser perforation bores holes on a print circuit board by scanning a short pulse laser beam of a high repetition rate in two dimensional directions by an X-galvanomirror and a Y-galvanomirror at a high speed, converging the scanned bear on the print board by an f.theta. lens, burning small regions vertically by the high power density and forming holes. Since light has neither mass nor inertia, the scanning rate of light beam is high enough. The galvanomirrors have some weights which would limit the scanning rate. The weight of the galvanomirror, however, can be alleviated. The laser perforation processing has a strong point of high perforation speed. Another advantage is the ability of boring a tiny hole of an under 100 .mu.m diameter.
Object materials are epoxy resins, polyimides and other resins which can make circuit boards. The lasers for optical perforation are mainly CO.sub.2 lasers. The light of a wavelength of 10.6 .mu.m is adopted as usual. But another wavelength, e.g., 9 .mu.m is sometimes used for enhancing boring performance for some materials. In the case of a YAG laser, a wavelength of 1.06 .mu.m will be adopted. But this description will explain the case of the CO.sub.2 laser light source of 9 .mu.m to 10.6 .mu.m of wavelength. The f.theta. lens for perforating holes on printed circuit boards is entirely different from the ordinary f tan .theta. lens which has been used in the laser processing, i.e., welding, cutting or annealing. The f.theta. lens is an extraordinary lens. This invention tries to propose a new f.theta. lens for the printed circuit board perforation.